The top three requests I hear from customers regarding WPF controls are something like the DataGridView in WinForms, Office 2007 Ribbon, and behavior similar to how Visual Studio allows you to drag & drop / dock / undock windows like the toolbox, etc.  Of course, all of this is possible with WPF and some effort.  However, there aren't any controls out of the box that make it almost effortless.  You need to look to a control vendor if you want to save some time.  Dennis Basaric from DevComponents informed me that they have released DotNetBar for WPF which includes their new Wpf-Dock control plus their existing Wpf-Ribbon control.  I haven't had a chance to work with the latest control yet, but it's definitely something I would evaluate if I needed docking behavior in my application.  Details here.

-Marc